Question: In chemistry class, the girl to boy ratio is $9$ to $6$. If there are a total of $75$ students, how many girls are there?
Explanation: A ratio of $9$ girls to $6$ boys means that a set of $15$ students will have $9$ girls and $6$ boys. A class of $75$ students has $5$ sets of $15$ students. Because we know that there are $9$ girls in each set of $15$ students, the class must have $5$ groups of $9$ girls each. There is a total of $45$ girls in chemistry class.